This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Monoamine oxidases ( A &B) are outer mitochondrial membrane-bound enzymes that play important roles in amine neurotransmitter metabolism. Sequence and modeling studies suggest human MAO A differs from all other mammalian MAO A in that it is monomeric rather than dimeric in the outer membrane due to a Glu151Lys mutation. Crystal structures of purified human and rat MAO A provide additional support for this hypothesis. To provide a direct experimental test, analogues of the inhibitor pargyline with a nitroxide spin label on the para position will be synthesized and covalently incorporated into the active sites of human and rat MAO A, the Lys151Glu mutant of human MAO A, and human MAO B. Structural data of the dimeric form of crystalline human MAO B show the distance between the active site cavities is 42 [unreadable]. This distance is readily determined by Double Quantum Coherence EPR measurements on these samples. The dipolar spectral data will provide distance information that will determine if these enzyme species are in their dimeric or monomeric forms in the mitochondrial outer membrane.